Hi, I don't know if anyone can help me here, but basically if you have a consumer unit on a house which a 100A fuse, does anyone know what the bottom line is as far as load goes? For example say two ring mains on 30 amps each cooker on another 30 amp, garage on 15, lighting 2x5 etc, eg where the total load is more than the main fuse, is there a limit as to how far you can go as the law sees it? I ask as I want to fit a shower and this is 9.5kw, now the total system as I calculate it should not exceed 100amps, but the total fuseable load may be 150A plus. Any suggestions especially with ref to any UK regulations would be welcome :)
There are no firm rules here, but the IEE On-Site Guide (which is not statutory) contains some recommendations. You should be able to buy this in any good bookshop. The section you want is the Appendix which covers Diversity.
For example, for the ring circuits, you assume 100% of one plus 40% of all others. For lighting circuits, you assume 66% of the total demand, where the total demand is 100W per lampholder (or actual value if lamp greater than 100W). For the shower, you have to assume
100% loading. (These figures are for household installations -- other types of installation have different guideline figures.)
So, using the figures you gave (and assuming worse case of 5A per lighting circuit as you didn't give enough info, and assuming no other circuits you didn't mention), that comes out to:
100% x 30A + 40% x 30A + 5A + 5A + 41A = 93A So this would be OK on 100A supply.
In practice nowadays, many houses fail to meet these guidelines due to splitting load into increasing numbers of circuits, failure of the guidelines to take into account energy saving lamps, etc. For example, I rewired a house a couple of years ago which originally had just 3 circuits, two 5A lighting circuits and a 30A ring circuit. After the rewire, the house now has 4 30A ring circuits because I wanted to include full RCD protection but without one earth fault losing the supply to all socket outlets (they are on separate RCBO's except one which supplies non-RCD protected appliances). I also put in a separate oven circuit, a separate circuit for a 2kW bathroom fanheater, and a separate circuit for the outside lights. Now if you work out the "before" and "after" supply requirements using the diversity guidelines, you will end up with vastly different figures. However, the actual load in the house has not changed by 1mA -- it still contains exactly the same appliances it contained before. So the diversity rules should not be applied without also paying some attention to the actual loads, and this might result in either lowing or raising the supply figure you work out using the guidelines.
In the UK, 100A is the maximum single-phase supply obtainable. If you really need more than 100A, you will have to upgrade to a 3-phase supply.
I got involved with a job in Manchester (luxury apartments), and they required a 130A supply (after diversity calcs), as everything in the flat was electric. We specked a160A SP supply to each flat fed from a TP busbar trunking riser, we had to spec a 200A TPN Type B dist board strapped out to SP.
They were big apartments, More than a humble EE can afford, not to mention paying the elecy bill!!
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